The present invention relates to castor wheel assemblies, in particular but not exclusively, to castor wheel assemblies of a type suitable for use with trolleys, for example supermarket trolleys or luggage trolleys.
It is known to fit castor wheels, in particular swivel castor wheels, to a variety of vehicles including trolleys or carts for use in warehouses, hospitals and retail establishments for use in transporting goods or persons.
In order to provide a high level of maneuverability, swivel castor wheel assemblies are typically provided at each of four corners of the vehicle so that the vehicle may be pushed in any direction over a floor. For example, a vehicle may be pushed in a forward direction, a backward direction, or sideways. The problem exists, however, that such vehicles lack directional stability. This can lead to difficulties in maneuvering a vehicle such as a shopping trolley or cart, which may in turn lead to accidental collisions and possible injuries.
It is helpful to consider the geometrical factors involved. In the field of motorcycle engineering, it is usual to speak in terms of the rake and the trail of the front steering system. Rake is the angle measured between the steering axis and vertical. The steering axis is the line about which the steering system turns. Trail is the distance measured from where the steering axis meets the ground to where a vertical line drawn though the front axle meets the ground. It can be thought of as the distance that the front wheel “trails” the steering axis.
The effects that rake and trail have on steering performance can be considered using a shopping trolley front wheel as an example. The front wheel of a shopping cart is a castor that has a vertical steering axis that is in front of the wheel. The vertical steering axis results in zero rake, and having the pivot in front of the wheel results in a significant amount of trail. This results in the front wheel tracking directly behind the pivot regardless of the direction the trolley is pushed. If the vertical pivot axis were directly above the wheel axle, the wheel would not track directly behind the pivot. In this case both the trail and the rake would be zero resulting in a wheel that has as much possibility of turning sideways as it does going straight. This is a very unstable condition for a shopping trolley.
It is known, for example in the field of automotive engineering, that mounting the front or rear wheels of a vehicle in a rearwardly-raked configuration can improve tracking and directional stability. However, motor vehicles are generally designed for high speed travel in a forwards direction only, and do not have swivel castor wheel assemblies.
One possible way of improving forward directional stability of a supermarket trolley is therefore to arrange the rear swivel castors so that they have a degree of rearward rake as well as a degree of trail. However, if a swivel castor with trail is also raked, this causes problems when the steering angle is changed. This is because the steering axis is not normal to the surface over which the castor moves, and if there is a non-zero amount of trail, the axis of rotation of the castor wheel will move upwardly or downwardly with respect to the point of attachment of the castor wheel to the trolley. If the trolley is heavy or full of goods, this lifting effect can cause serious difficulties for the operator manipulating the trolley.